eclipse is actually 2 km shorter than the currently accepted length of the executor, 19km. the eclipse is a lot more massive than executor, on account of all of the extra systems installed, such as a reactor powerful enough for the superlaser to function, the weapon itself, as well as all of the additional systems like the gravity wells, etc. and i imagine that the armor and shields are immensely strong, so as to reduce the risk of destruction from the blast and debris that occur from irreparably damaging a planet, while practically stationary, and under fire